Bipolar crossed-field switch tube with uniform magnetic field

ABSTRACT

Crossed-field switch tube has two electrodes to define an annular interelectrode gap. An annular magnetic field is produced in the gap with the field uniform along the length of the gap. Circuit connects electrodes so that, with alternating polarity, the gap alternately conducts with about the same voltage drop.

Gallagher et al.

1 1 BIPOLAR CROSSED-FIELD SWITCH TUBE WITH UNIFORM MAGNETIC FIELD [75]Inventors: Hayden E. Gallagher; Wolfgang Knauer, both of Malibu, Calif.

[73] Assignee: Hughes Aircraft Company, Culver City, Calif.

22 Filed: Sept. 18, 1974 [21] Appl. No.: 507,095

[52] US. Cl. 313/157; 313/158; 313/161 1 l [58] Field of Search 313/157,158, 161. 162

[56] References Cited UNITED STATES PATENTS 3,641.384 2/1972 Lund elal, 1. 313/161 [4 1 Sept. 16, 1975 Primary Examiner-Ri V. RolinecAssistant ExaminerDarwin R. Hostetter Attorney, Agent, or FirmAllen A.Dicke, Jr.; W. H. MacAllister 9 Claims, 6 Drawing Figures [I Dr, I

PATENTED SEP 1 5 3975 SHKU 1 15 2 Fig. 2 PRlOR ART BIPOLAR CROSSED-FIELDSWITCH TUBE WITH UNIFORM MAGNETIC FIELD BACKGROUND OF THE INVENTION Thisinvention is directed to a single gap bipolar crossed-field switch tubeand circuit for particular use in circuits where conduction in eitherdirection is desired.

Bipolar conduction capability in a crossed-field switch tube isessential for all AC applications of the switch tube. Furthermore, it isdesirable also for DC breaker applications since, in multi-terminaltransmission systems, reversal of power flow is achieved mostconveniently by reversal of the current flow direction.

Crossed magnetic and electric field devices are known in the prior art.Perhaps the first disclosure of a crossed field device for switching isPenning US. Pat. No. 2,182,736. Boucher US. Pat. Nos. 3,215,893 and3,215,939 are primarily directed to crossed-field rectifier typeswitching and are directed to an improvement where the shape of themagnetic field is asserted to improve rectifying action by providing alower breakdown voltage in one direction than the other between the twoelectrodes which define the gas-filled space.

M. A. Lutz and R. C. Knechtli U.S. Pat. No. 3,838,061 is one of a seriesof patents which indicates modern developments for higher voltage offswitching and higher current capability. Other patents of this natureinclude G. A. G. Hofmann US. Pat. No. 3,604,977 and G. A. G. Hofmann andR. C. Knechtli US. Pat. No. 3,558,960. There are also other patentsdirected to improvements in the crossed-field switching device.

One particular patent which is pertinent background for the presentinvention is G. A. G. Hofmann and R. E. Lund US. Pat. No. 3,641,384which describes a crossed-field switching device which has three spacedelectrodes and two gas-filled annular spaces therebetween. That patentrepresents a structure which was for the purpose of higher voltagehold-off in series connection and higher current capacity in parallelconnection in DC applications. In other words, it was intended that bothgaps would be conducting and off-switched at the same time.

SUMMARY OF THE INVENTION In order to aid in the understanding of thisinvention, it can be stated in essentially summary form that it isdirected to a bipolar crossed-field switch which has two concentricelectrodes to form an annular interelectrode gap and means for producingan annular magnetic field which extends substantially uniformly throughthe length of the interelectrode gap so, with opposite application of anelectric field to the gap, opposite conduction takes place with aboutthe same voltage drop.

It is thus an object of this invention to provide a bipolarcrossed-field switch tube which is particularly arranged to conduct ineither direction. It is a further ob ject to provide a crossed-fieldtube which is connectable into a circuit of such nature that conductionin either direction can be required by the circuit and can be achievedby the switch tube together with off-switching of such conduction. It isanother object to provide a crossed-field switch tube with a single gapand having a magnetic field which is substantially uniform along thelength of the gap to permit electric conduction in either direction atsubstantially the same voltage drop.

It is yet another object to provide a bipolar structure in a singleenvelope for economy of space, manufacture and maintenance.

Other objects and advantages of this invention will become apparent froma study of the following portion of this specification, the claims, andthe attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic circuitemploying the bipolar crossed-field switch tube of this invention.

FIG. 2 is a longitudinal section through a schematic switch tubeexemplifying the prior art.

FIG. 3 is a longitudinal section through a switch tube illustratingbackground for this invention for producing a uniform magnetic field inthe interelectrode space.

FIG. 4 is a longitudinal section through a schematic switch tubeexemplifying the second preferred embodiment.

FIG. 5 is a longitudinal section through a switch tube exemplifyinganother preferred embodiment.

FIG. 6 is a longitudinal section through a switch tube exemplifyinganother preferred embodiment of the switch tube of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 2 illustratescrossed-field switch tube 10 which shows the relationship of the partsand characteristics as known in the prior art. Switch tube 10 comprisesan outer tubular cylindrical electrode I2 which is closed at the bottomand serves also as the enclosing tank of the device. Interelectrode I4is positioned within outer electrode 12 and defines an annularinterelectrode space or gap I6. Tubular insulator I8 engages around theupper reduced diameter neck 20 of outer electrode 12 and engages on disc22 upon which interelectrode I4 is mounted. Thus they are physicallysecured with respect to each other to maintain gap 16 and to enclose theinterior of the outer electrode so that the proper gas conditions can bemaintained in the gap 16. The prior art discussed in the backgrounddiscusses the gas conditions and other operating parameters of theswitch device and that enumerated prior art is incorporated herein inits entirety by this reference. Electric connections are made to the twoelectrodes so that they can be connected into an appropriate circuit.

A magnetic field is supplied to the interelectrode gap. The field may besupplied by a permanent magnet and switched by means of anelectromagnetic coil or the entire field may be suppliedelectromagnetically. In the present case, electromagnet 24 supplies afield 26 which is schematically illustrated in dashed lines.

Switch tube I0 is a crossed-field switch. When electric potential isapplied, the potential is radial across the interelectrode space. Theother field is the magnetic field provided by magnet 24. Duringconducton. both fields are on, and electrons spiral through the annularinterelectrode space due to the magnetic field action to providecascading breakdown, and thus conduction. For off-switching, themagnetic field is turned off so that the electron path is substantiallyradial, too short to cause cascading breakdown.

Prior art crossed-field switch tubes employed the outer electrode as thecathode, because the glow discharge is cathode area-limited rather thananode arealimited. With the outer electrode of larger area, this was thenatural connection polarity. With the curved nature of the magneticfield 26, as illustrated in FIG. 2, there is axial concentration of theelectrons during conduction so that the voltage drop is higher when theelectrode 14 is connected as cathode than when electrode 12 is connectedas cathode. This was originally thoL g. t to be a function of thecathode area-limiting factor, but it has now been discovered that thediffer' ence in voltage drop in the two directions is a function of themagnetic field shape. This invention is directed to shaping the magneticfield in the gap so that there is substantially no axial shaping so thatthe voltage drop is substantially equal for conduction in eachdirection. With an axial magnetic field, voltage drop in eitherdirection is about 500 volts for conduction of amperes per squarecentimeter. With the prior art magnetic field, the voltage drop with theouter electrode 12 connected as cathode was 300 volts for conduction of10 amperes per square centimeter while with the interelectrode 14connected as cathode was about 1,000 volts for conduction of 10 amperesper square centimeter. Current density figures are calculated using thecathode area.

The circuit of FIG. 1 is a schematic AC current limiter generallyindicated at 30. Terminal 32 is connected to the line, and terminal 34is connected to the load. Mechanical switch 36 is normally closed sothat current normally flows therethrough. When a fault occurs andcurrent rises, current limiter circuit is operated to hold the currentdown to tolerable levels until the fault is cleared or the circuit isopened by normal circuit breakers. When a fault is sensed, switch 36 isopened and switch tube 50 conducts the alternating current with the gapalternatly conducting so that the AC current is conducted until switch36 is opened and deionized. Thereupon, the magnetic field is switchedoff of switch tube 50 so that it becomes non-conductive. This switchessurge-limiting capacitor 38 and currentlimiting impedance 40 into thecircuit to hold down the circuit current to tolerable levels, preferablysubstantially to full load levels. Current is thus limited until thefault is cleared or the main circuit breakers open the line.

Crossed-field switch device in FIG. 3 comprises inner electrode 52 andouter electrode 54 which define the interelectrode gap 56. Theelectrodes are supported as in the manner of FIG. 2 and are surfaces ofrevolution around the center axis 58 of switch 50. Magnet 60 provides amagnetic field in the gap. Insulators 62 and 64 are positioned at theend of the gap and carry auxillary electrodes 68 and 70 which causetrapping of the electrons between the axial ends of the gap. Aselectrically floating electrodes 68 and 70 are negatively charged up bythe first electrons leaking out of the trapped region, the electrodevoltage repels or traps the remaining electrons in the interactionregion. In this way, a uniform field is produced together with an axialelectron trapping to result in conduction in either direction at areasonable voltage drop, for example 500 volts for a conduction of 10amperes per square centimeter of cathode area. The use of anelectrically floating electrode to encourge axially electron trapping toaid in the initiation and maintainance of the plasma discharge isbackground to this invention.

FIG. 4 illustrates a preferred embodiment 72 of this invention. lt hasinner electrode 74 and outer electrode 76 defining gap 78. Magnet 80produces flux in the gap.

In crossed-field switch 72, magnetic shims 82, 1, 86 and 88 arepositioned to direct the magnetic flux along "m s 96. The magnetic shimsare of magnetic material such as soft iron configured as annular ringspositioned interiorly and exteriorly of the gap at its axial ends todirect the flux generally axially of the gap. With these magnetic shims,it appears that the flux passes from the center of the gap in an axialdirection out through the shims so that the flux is concave in adirection toward both of the electrodes. This permits axial electrontrapping while the switch tube is conducting to limit voltage drop. Inthis configuration, voltage drop in either direction of conduction isabout 500 volts at a current of 10 amperes per square centimeter ofcathode area. With the positioning of the pole pieces outside of theplasma gap, the voltage holdoff value is not compromised. Electrontrapping by the curvatures of the magnetic field takes place only in aportion in the radial direction of the interaction gap. In the otherradial portion of the interaction gap, the magnetic field curvature isin the wrong direction for electron trapping; however with properdimensioning, the magnetic field curvature in the correct direction isadequate for adequate trapping. It is the simplest structure, because itleaves the interaction gap unencumbered by insulators or magnetic polepieces which might interfere with the plasma or the interelectrodevoltage breakdown value.

Crossed-field switch 92 of FIG. 5 has an inner electrode 94 and an outerelectrode 96 which define the interelectrode gap 98. Magnet 100 providesa magnetic field to the gap, with the flux lines indicated at 102. Inthis embodiment of the crossed-field switch, soft iron pole pieces ormagnetic shims 104 and 106 are posi tioned at the axial ends of theannular gap. Shims 104 and 106 are rings of soft iron or other magneticmaterial to direct the flux. When rings of the appropriate size andmaterial are properly positioned, the magnetic flux lines 102 aresubstantially axial through gap 98. Magnetic shims 104 and 106 areelectrically separated from both of the electrodes and thus areelectrically floating in the space to charge up and provide electrontrapping. With this configuration, about the same voltage drop isachieved as with the configurations in FiGS. 3 and 4.

Crossed-field switch device 110 in FIG. 6 is also of the same generalconfiguration. Inner electrode 112 faces outer electrode 1 14 to defineannular gap 116. ln this case, two magnets (magnets 118 and 120) providethe magnetic field in the gap, as illustrated by flux lines 122. In eachof the switch tubes in FIGS. 3 through 6, pressure is controlled in thegap, and magnetic field is supplied by an electromagnet or anelectromagnet plus a permanent magnet. By switching the electromagnet,the magnetic field in the gap can be changed sufficiently to causeoff-switching as described above. In this structure, electron trappingoccurs in one polarity by curvature of the magnetic field byenergization of one solenoid 118 or 120 and occurs for the other polarity by energization of the other solenoid.

There are two methods of operation of the crossedfield switch 110 ofFIG. 6. in one method of operation, both magnets 118 and 120 are on atthe same time so that an adequate flux, as represented by line 122, isproduced in the gap to permit conduction. Since there are two magnets,the flux lines are substantially symmetrical through the gap so thatconduction in either direction has about the same characteristics. Thus,

voltage drop in the two directions is substantially equal. Foroff-switching, one or both of the magnets are turned off. The magnetscan be sized so that both must be on for conduction so that. when one isturned off, off-switching occurs. For rapid off-switching, preferablyboth are provided with a bucking magnetic field for off-switching. As analternate method of operation, magnets 118 and 120 can be alternatelyenergized for switching, depending upon the direction of desiredconduction. When operated in this way, a lower voltage drop isachievable, but selective operation of the two magnetic fields isrequired, in response to the impressed polarity of the electric field.

This invention having been described in its preferred embodiment, it isclear that it is susceptible to numerous modifications and embodimentswithin the ability of those skilled in the art and without the exerciseof the inventive faculty. Accordingly, the scope of this invention isdefined by the scope of the following claims.

What is claimed is:

l. A crossed-field switch device comprising:

a cylindrical inner electrode, a hollow cylindrical outer electrodespaced around said inner electrode to define an annular gap in which aglow discharge cascading breakdown can be maintained, said gap having alengthwise direction parallel to the axis of said electrodes, means formaintaining a subatmospheric gas pressure in said gap, the improvementcomprising:

means for applying a potential at either polarity to said gap;

Magnetic field means for producing a magnetic field which issubstantially axial of said gap in the breakdown region of said gap sothat either electrode can act as cathode with substantially the samevoltage drop because of substantially the same amount of electrontrapping in the breakdown region of said gap with application ofpotential of either polarity.

2. The crossed-field switch device of claim 1 wherein said magneticfield means includes a magnet positioned exteriorly of said outerelectrode adjacent said gap for inducing a magnetic field in said gap.

3. The crossed-field switch device of claim 2 wherein said gap has aline of symmetry therethrough along the length thereof and substantiallyequally spaced from said electrodes, said magnetic field beingsubstantially symmetrical on opposite sides of said line of symmetry.

4. The crossed-field switch device of claim 3 wherein said magneticfield means includes a magnetic pole piece at each end of said gappositioned interiorly of said outer electrode and along the line ofsymmetry.

5. The crossed-field switch device of claim 2 wherein said magneticfield means also includes a magnet positioned within said innerelectrode so that the net magnetic field resulting from said outermagnet and said inner magnet is symmetrical along the line of symmetry.

6. The crossed-field switch device of claim 4 wherein at least one ofsaid pole pieces is electrically separated from both of said electrodes.

7. The crossedfield switch device of claim 6 wherein both of said polepieces are electrically separated from both of said electrodes.

8. The crossed-field switch device of claim 4 wherein said magnetic polepiece is mounted on said inner electrode at each end of said gap forforming the magnetic field symmetry.

9. The crossed-field switch device of claim 8 wherein there is also amagnetic pole piece mounted on said outer electrode adjacent each end ofsaid gap for shaping the magnetic field.

1. A crossed-field switch device comprising: a cylindrical innerelectrode, a hollow cylindrical outer electrode spaced around said innerelectrode to define an annular gap in which a glow discharge cascadingbreakdown can be maintained, said gap having a lengthwise directionparallEl to the axis of said electrodes, means for maintaining asubatmospheric gas pressure in said gap, the improvement comprising:means for applying a potential at either polarity to said gap; Magneticfield means for producing a magnetic field which is substantially axialof said gap in the breakdown region of said gap so that either electrodecan act as cathode with substantially the same voltage drop because ofsubstantially the same amount of electron trapping in the breakdownregion of said gap with application of potential of either polarity. 2.The crossed-field switch device of claim 1 wherein said magnetic fieldmeans includes a magnet positioned exteriorly of said outer electrodeadjacent said gap for inducing a magnetic field in said gap.
 3. Thecrossed-field switch device of claim 2 wherein said gap has a line ofsymmetry therethrough along the length thereof and substantially equallyspaced from said electrodes, said magnetic field being substantiallysymmetrical on opposite sides of said line of symmetry.
 4. Thecrossed-field switch device of claim 3 wherein said magnetic field meansincludes a magnetic pole piece at each end of said gap positionedinteriorly of said outer electrode and along the line of symmetry. 5.The crossed-field switch device of claim 2 wherein said magnetic fieldmeans also includes a magnet positioned within said inner electrode sothat the net magnetic field resulting from said outer magnet and saidinner magnet is symmetrical along the line of symmetry.
 6. Thecrossed-field switch device of claim 4 wherein at least one of said polepieces is electrically separated from both of said electrodes.
 7. Thecrossed-field switch device of claim 6 wherein both of said pole piecesare electrically separated from both of said electrodes.
 8. Thecrossed-field switch device of claim 4 wherein said magnetic pole pieceis mounted on said inner electrode at each end of said gap for formingthe magnetic field symmetry.
 9. The crossed-field switch device of claim8 wherein there is also a magnetic pole piece mounted on said outerelectrode adjacent each end of said gap for shaping the magnetic field.